Many enclosures that contain sensitive equipment must maintain very clean environments in order for the equipment to operate properly. Examples include enclosures for the following: optical surfaces or electronic connections that are sensitive to particulates and gaseous contaminants which can interfere with mechanical, optical, or electrical operation; data recording devices, such as computer hard disk drives that are sensitive to particles, organic vapors, and corrosive vapors; processing and storage of thin films and semiconductor wafers; and electronic controls such as those used in automobiles and industrial applications that can be sensitive to particles, moisture buildup and corrosion as well as contamination from fluids and vapors. Contamination in such enclosures originates from both inside and outside the enclosures. For example, in computer hard drives, damage may result from external contaminates as well as from particles and vapors generated from internal sources. The terms “hard drives” or “hard disk drives” or “disk drives” or “drives” will be used herein for convenience and are understood to include any enclosure for equipment or material that is sensitive to contamination.
Disk drives must be protected against a large number of contaminants that are found in the surrounding environment and can penetrate the drive. This is particularly true for drives that are removable and portable to any environment such as disk drives that are used in laptop computers or in Personal Computer Memory Card International Association (PCMCIA) slots, or other drives which may not be used in the typical data processing environment. Drives used in applications such as gaming systems, personal video recorders, automotive mapping systems and others must survive in environments that are more severe than that of standard desk top computer applications.
Contamination may occur in various forms. For example, disk drives are susceptible to corrosive ions, such as chlorine and sulfur dioxides, and may also be sensitive to variations in humidity. Accordingly, an array of failure mechanisms exist.
One serious contamination-related failure mechanism in computer disk drives is static friction or “stiction.” Stiction results from the increased adhesion of a drive head to a disk while the disk is stationary plus increased viscous drag parallel to the head-disk interface. Newer high-density disks are more sensitive to contamination-caused stiction because they are smoother and include relatively thin layers of lubricants. Contaminants on the disk change the surface energy and increase the adhesive forces between the head and disk, causing stiction. Also, stiction may be caused by vapors condensing in the gap between the head and disk. The low energy low torque motors that are being used in smaller disk drives for portable computers and the low noise drives used in other applications, such as personal video recorders, are increasingly sensitive to stiction related failures.
Another serious contamination-related failure mechanism is a head crash. Head crashes can occur when particles get into the head disk interface. The spacing or flying heights between the head and disk during operation of modern high density drives is 30 nanometers or less. As rotational speed affects the maximum data transfer rate a drive can have, rotational speed of modern disk drives is increasing. Some current drives operate at 15,000 revolutions per minute and future drives will likely use even higher speeds. With such high speeds and low flying heights, even submicron-sized particles can be a problem, causing the head to crash into the particle or the disk after flying over a particle, bringing the drive to an abrupt failure mode. Particles can also adversely affect data integrity and mechanical reliability of a drive, sometimes referred to as thermal asperity.
Disk Drives are also susceptible to variances in humidity. Low humidity is problematic either because it may increase static electricity or decrease lube thickness or functionality. However, in high humidity, corrosion is promoted and lubricants may swell. It takes significantly more adsorbent to protect a drive from humidity than it does from organic or acid gas contamination. Thus drives that need buffering from humidity fluctuations require significant amounts of adsorbent.
To prevent contamination-related failure, a variety of filtration devices have been used. For example, filtration devices to keep particles from entering disk drives are well known. Some consist of a filtration media held in place by a housing of polycarbonate, acrylonitrile butadiene styrene (ABS), or some other material. Others consist of a filtration media in the form of a self-adhesive disk utilizing a layer or layers of pressure sensitive adhesive. Such filters are mounted and sealed over a vent hole in the enclosure to filter particulates from the air entering the drive. Filtration performance depends not only on the filter media having a high filtration efficiency but also on having it have a low resistance to airflow. If the pressure drop is too high, unfiltered air will leak into the enclosure through a gasket, screw hole, or other seam instead of entering through the filter. Such filters may work well for particulates of external origin, but do not address the problems from vapor phase contaminants.
Internal particulate filters, or recirculation filters, are also well known. These filters are typically pieces of filter media, such as expanded PTFE membrane laminated to a polyester nonwoven backing material. Other recirculation filters are “pillow-shaped” filters containing electret (i.e., electrostatic) filter media. These filters may be pressure fit into slots or “C” channels and are placed in an active air stream such as near the rotating disks in a computer hard disk drive or in front of a fan in electronic control cabinets, etc. Alternatively, the recirculation filter media can be held in a plastic frame. Still alternatively they can be applied to the sides of components or the housing to allow for particle collection. Recirculation filters work well for particulate removal of internally generated particles but do not address the problem of vapor phase contaminants, nor do they provide protection from external particles entering the drive.
Internal adsorbent filters are also well known. One example is described in U.S. Pat. No. 4,830,643 issued to Sassa et al. This patent teaches a sorbent filter where a powdered, granular or beaded sorbent or sorbent mixture is encapsulated in an outer expanded PTFE tube. This filter is manufactured by W. L. Gore & Associates, Inc., Elkton, Md., and is commercially available under the trademark GORE-SORBER® module. A second well known internal adsorbent assembly is described in U.S. Pat. No. 5,593,482 issued to Dauber et al. A third internal adsorbent assembly incorporates a layer of adsorbent such as activated carbon/PTFE composite between two layers of filter media or is alternately wrapped in a layer of filter media and can be installed between slots or “C” channels much the way a recirculation filter is installed but without significant airflow through the filter. Such a filter is described in U.S. Pat. No. 5,500,038 issued to Dauber et al.
Known internal adsorbent filters work well at adsorbing vapor phase contaminants, but they do not filter particulates very well. They may collect particles by some impaction of particles onto the filter (i.e., by having the larger particles impacting or colliding with the adsorbent filter as particle-laden air speeds around the filters) or by diffusion of particles onto the filter. However, these filters do not perform nearly as well as the standard recirculation filters, which work by a combination of sieving (mechanically capturing particles too large to pass through the pore structure of the filter), impaction (capturing particles too large to follow the bending air streams around filters or the fibers of the filter), interception (capturing particles that tend to follow the air streams, but are large enough to still intercept a filter fiber or in other words those particles with a diameter equal to or less than the distance between the fiber and the air stream line), and diffusion (capturing smaller particles buffeted about by air molecules in a random pattern and coming into contact with a filter fiber to become collected).
Because there is a need to remove vapor phase contaminates as well as particles from both internal and external sources, combination sorbent breather filters were developed. These can be made by filling a cartridge of polycarbonate, ABS, or similar material with sorbent and securing filter media on one or both ends of the cartridge and placing the cartridge over a hole in the container wall. These filters effectively cleanse incoming air of particles and vaporous contaminates, and internal air of internally generated vaporous contaminates. Because the filters are inside, the vaporous drive contaminate will diffuse into the adsorbent sections of the filters. Examples of such filters are described in U.S. Pat. No. 4,863,499 issued to Osendorf (an anti-diffusion chemical breather assembly for disk drives with filter media having a layer impregnated with activated charcoal granules); U.S. Pat. No. 5,030,260 issued to Beck et al. (a disk drive breather filter including an assembly with an extended diffusion path; U.S. Pat. No. 5,124,856 issued to Brown et al. (a unitary filter medium with impregnated activated carbon filters to protect against organic and corrosive pollutants); and U.S. Pat. No. 5,447,695 issued to Brown et al. (Chemical Breather Filter Assembly). Unfortunately, many of these designs are too large and take up too much space in today's miniaturized drives.
To adsorb corrosive compounds such as chlorine and sulfur dioxide, an adsorbent is typically treated with a salt to chemisorb the contaminants. However, when many known filters are washed in deionized water, large amounts of these salts may be released, which makes them unacceptable in sensitive disk drive environments.
A washable adsorbent recirculation filter is described in U.S. Pat. No. 5,538,545 issued to Dauber et al., wherein expanded PTFE membranes or other hydrophobic materials are used to encapsulate the adsorbent. However, these filters do not filter air as it comes into the drive before it has had a chance to deposit particles and do damage to the drive.
A second combination adsorbent breather filter is also well known that encapsulates the adsorbent material such as an impregnated activated carbon polytetrafluoroethylene (PTFE) composite layer between two layers of filter media and is applied over a hole in the enclosure with a layer of pressure sensitive adhesive. These filters work well to an extent and are of a size that can be used in today's small drives and are typically designed to filter air coming into the drive. Thus, the adsorbent is typically primarily designed to adsorb both organic and corrosive vapors from the outside environment and will filter particulates only from air coming into or leaving the drive. Internally generated vapors and moisture can be adsorbed by these filters, but often times they have been used in conjunction with another larger internal adsorbent filter so the adsorbent breather filter can be smaller in size. Therefore, such filters may not contain enough adsorbent to adequately adsorb all the internally generated contaminants and typically will not contain enough adsorbent to control humidity well within the drive as previously mentioned. Again, particles are also generated inside the drive and are not typically captured by these filters.
Combinations of several filters having different functions in a single drive have been taught. For example, U.S. Pat. No. 5,406,431, to Beecroft, describes a filter system for a disk drive that includes an adsorbent breather and recirculation filter in specifically identified locations within the drive. Also, U.S. Pat. No. 4,633,349, by Beck et al., teaches a disk drive filter assembly comprising a dual media drum type filter element in a recirculating filter assembly that surrounds a breather filter. Further, U.S. Pat. No. 4,857,087, to Bolton et al., teaches incorporating a breather filter in a recirculation filter housing, but has significantly more parts and incorporates a third filter element complete with housings, apertures, and gaskets to accomplish this inclusion. The combinations described in these patents either locate the filter components in separate regions of the disk drive or incorporate space-consuming fixtures to orient the component parts within the drives.
Space saving combinations having further multifunctionality have also been taught. These include U.S. Pat. No. 6,266,208 to Voights integrating a recirculation filter, breather filter, and adsorbent filter into a single unitary filter; U.S. Pat. No. 6,238,467 to Azarian et al., incorporating a breather filter, adsorbent filter, and recirculation filter into a rigid assembly filter; U.S. Pat. No. 6,296,691 to Gidumal incorporating a breather filter adsorbent filter and recirculation filter into a molded filter; and U.S. Pat. No. 6,395,073 to Dauber incorporating the recirculation filter and breather filter with optional adsorbent filter into a low profile adhesive construction. All but the last filter design by Dauber are filters of considerable size and are not appropriate for smaller drives like the 2.5″ laptop drives and smaller 1.8″ drives, 1.0″ drives and 0.85″ drives that are currently in the market and/or in design.
As disk drives have become smaller and less expensive, there is a need for simplification and the reduction in the number of parts in a drive to reduce cost and improve performance. However, as the drives increase in recording data density and capacity, they become more sensitive to particulate and vaporous contamination including increased sensitivity to moisture.
Existing filtration means often do not meet these demanding filtration requirements. The low profile adsorbent breather filters and low profile multifunction filters best suited in size to fit these smaller drives have had to compromise in adsorbent content because they needed airflow through the filter. The compromise has been to either use very dense adsorbent media and have limited airflow, or use less dense adsorbent media to maintain airflow but then have limited adsorbent capacity. Two recent inventions have tried to overcome this deficiency. U.S. Pat. No. 6,683,746 to Kuroki et. al. allows for air by-pass of the adsorbent media to increase airflow, but can have performance reductions in adsorption if too much air by-passes the adsorbent media. U.S. Pat. No. 6,712,887 to Ueki et. al. uses grooves in the adsorbent media to increase airflow, but this has limited functionality as there are further limitations to airflow through the filtration layer unaddressed by this invention.